Multilevel inverters (such as a five level inverter) are receiving increased attention in both academia and industry as an optimal solution for power conversion for medium and high power applications. For medium power (such as 50-100 kW output) and high power (such as 500-1,000 kW output, or higher) applications, the motivation for the use of multilevel inverters is to reduce switch voltage stress as well as output filter size. Multilevel inverters also have the advantages of improved output quality, lower Total Harmonic Distortion (THD), lower common-mode voltage, and lower Electromagnetic Interference (EMI) as compared to their three-level and two-level counterparts. Furthermore, multilevel inverters can achieve higher efficiency as they can use lower voltage rating devices, leading to their wide range of application fields, including renewable energy such as photovoltaic (PV) inverters, wind-powered generators, and so on.
There are three types of multilevel inverter topologies: Neutral Point Clamped (NPC) type, Flying-Capacitor (FC) type, and Cascaded H-Bridge (CHB) type. Hybrid multilevel inverter topologies combine features of NPC and FC. Among hybrid topologies, the Five-Level Active Neutral Point Clamped (5L-ANPC) inverter provides an acceptable compromise between cost and performance. The 5L-ANPC inverter combines a 3L-ANPC leg with a 3L-FC power cell. The number of levels is increased with the levels introduced by the FC. This topology enables a modularity factor that is lacking in the NPC type inverter by adding the FC to reach the higher level without adding series-connected diodes. In addition, ANPC inverters split the DC-link into two capacitors, so the complexity of DC-link capacitor voltage balancing is reduced as compared to the conventional NPC and FC type inverters which require four DC capacitors in series. Due to the reduced costs, volume, and control complexity, the 5L-ANPC inverter is receiving more attention recently and is already commercially used for medium power level industrial applications. FIG. 1 shows a typical 5L-ANPC inverter topology [1]. It is noted that eight switches (i.e., 8S) in each phase are needed for this circuit. Therefore, for a three phase inverter, 24 switches are needed.
The redundant switching states in ANPC inverters allow the voltage across the FC to be regulated. To generate the switching pulses and simultaneously regulate the FC voltage, a variety of modulation strategies have been presented such as carrier-based Pulse Width Modulation (PWM), modified triangular carrier-based PWM, real time THD minimization, and selective harmonic elimination PWM [2]. The modulation of a conventional five-level ANPC inverter under PF=1 and PF<1 is shown in FIGS. 2A and 2B, respectively. It is noted that the leg voltage (VAO) is selected based only on the required output voltage. It is not dependent on the output current under either PF=1 or PF<1 condition. For example, it is observed from FIG. 2B that from t0 to t1, the instantaneous output current is negative, and the leg voltage is changed between 0V and Vdc/4. From t1 to t2, the instantaneous output current is positive, and the leg voltage is also changed between 0V and Vdc/4. This is due to the fact that the instantaneous output voltage Vref is less than Vdc/4 from t0 to t2. A similar observation is made from t4 to t6.
A six-switch 5L-ANPC (6S-5L-ANPC) inverter topology has been proposed, as shown in FIG. 3 [3]. As compared with the conventional 8S 5L-ANPC inverter, as shown in FIG. 1, the 6S-5L-ANPC inverter uses six switches for each phase and 18 switches for three phase application. Therefore, the total number of switches is reduced and the total inverter cost can be reduced.
The inverters are required to provide active power and reactive power to the grid. If the inverter's output current and output voltage are at same phase, the polarities of the output current and output voltage are always the same (either both are positive or both are negative), only the active power is provided to the grid. In this case, the power factor (PF) is one, PF=1. The top waveforms of FIG. 6 show the condition when the inverter output current and output voltage are at same phase. In FIG. 6, in zone Z1, the inverter's output current and output voltage are both positive and in zone Z2, the inverter's output current and output voltage are both negative.
If the power factor is less than 1, PF<1, the inverter's output current and output voltage are at different phases, and for some portion of the 60 Hz period, (or 50 Hz in Europe and far east countries), the polarities of the output current and output voltage are not the same (either positive current and negative voltage, or negative current and positive voltage). For the rest of the 60 Hz period, the polarities of the output current and output voltage are the same (either positive current and positive voltage or negative current and negative voltage). The top waveforms of FIG. 7 show the condition when the output current lags the output voltage by ϕ degrees. In FIG. 7, in zone Z1, the inverter's output current and output voltage are both positive and in zone Z2, the inverter's output current and output voltage are both negative. In zone Z3, the output current is negative and the output voltage is positive. In zone Z4, the output current is positive and the output voltage is negative. In zone Z3 and Z4, the output current and output voltage are in opposite polarity.